Power amplifier nonlinearities have a significant impact on the overall performance of modern digital communication systems.
The AM-PM (amplitude modulation to phase modulation) non-ideal transfer characteristic plays a predominant role amongst power amplifier non-idealities. A great deal of effort is invested in complex baseband/digital pre-distortion schemes to sense and counteract its effects. For such schemes to be efficient and viable, however, the spread of AM-PM should be minimized in the analogue domain in the first place. Furthermore, this should ideally be achieved across the whole frequency range of operation, especially for power amplifiers that are required to operate over a very wide frequency range.
A further non-linearity is variation of the power amplifier gain with temperature. In order to ease the requirements placed on the rest of the transmit-chain blocks, such variation should be kept as small as possible.